This invention relates to power-assisted steering gear which is intended for use in road vehicles and which is of the kind including a fluid-pressure actuated device which is controlled by a valve that is itself actuable in use by the driver of a vehicle in which the steering gear is mounted.
The resistance felt by the driver of a road vehicle having a manually operable steering gear as the steering wheel of the vehicle is turned varies from a relatively large resistance when the vehicle is parked or travelling at a very low speed to a relatively small resistance when the vehicle is travelling at high speeds. It is therefore desirable to arrange, if possible, when power-operated assistance is used, that the assistance given to manual operation of the steering gear should similarly vary from a maximum when the vehicle is stationary or travelling at low speeds to a minimum when the vehicle is travelling at high speeds. This desideratum could be achieved for example in an hydraulically actuated steering gear having a hydraulic pump by arranging that the hydraulic pressure available to operate the gear shall decrease as the vehicle road speed increases. Such an arrangement would however result in the disadvantage that at high road speeds the steering would develop, so far as the driver is concerned, a considerable amount of "sloppiness" and in the last resort when the hydraulic pressure had reached a very low or zero value, steering control would be exercised or substantially exercised by a direct mechanical connection through inter-engaging dogs which are normally provided, for safety reaons in the event of loss of hydraulic fluid, on two relatively movable valve members, such dogs however providing a substantial amount of "lost motion" in operating the steering gear.